JP2000098372A - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the polarized light conversion efficiency while improving the front face luminance by directivity to respond to requirement toward a low power consumption and high luminance. SOLUTION: In the liq. crystal display device arranging a liq. crystal display element 10 controlling polarization and an illuminator on the back face of a liq. crystal display element 10, a reflection means to the above described illuminator, an optical path conversion means 30 and a reflection type polarizing selection means (reflection type polarizing plate) 20 between the liq. crystal display element 10 and the above described illuminator are provided. And, an angle formed by an optical path conversion axis 31 of the optical path conversion means 30 and a polarizing transmission axis 21 of the reflection type polarizing selection means 20 is specified in the range of 0-30 deg. or 60-90 deg. so that transmitted light of the prescribed reflection type polarizing selection means 20 and a polarized state of reflected light been incident again on the reflection type polarizing plate 20 reflected by the above described reflection means become almost the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high brightness liquid crystal display device, and more particularly to a liquid crystal display device having high polarization conversion efficiency.

[0002]

2. Description of the Related Art In recent years, the technical progress of liquid crystal display devices, especially color liquid crystal display devices, has been remarkable, and many displays having display quality not inferior to that of CRTs have come to be seen. Furthermore, notebook personal computers have become widespread, and do not function as a display without a backlight as a lighting device. Therefore, a backlight is an essential device in a direct-view color liquid crystal display device.

The liquid crystal display devices are roughly classified into two types: a TN (twisted nematic) liquid crystal display device by active matrix driving using a TFT (thin film transistor) and a STN (super twisted nematic) liquid crystal display device by multiplex driving. . In both cases, polarizing plates are arranged on both sides of an element in which a liquid crystal layer is held by a glass substrate, and display is performed by modulating the polarization state of linearly polarized light.

[0004] The brightness level required of these liquid crystal display devices varies depending on the application. Particularly, in a notebook personal computer, not only the required brightness but also thinness, light weight and low power consumption are paramount. Furthermore, there is a high expectation for a large screen display as a display device such as a desktop computer or a workstation, and a display with low power consumption and high brightness is required.

Therefore, in order to realize an improvement in brightness, a polarization conversion using a cholesteric liquid crystal is disclosed in
No. 45906. Further, a configuration in which a cholesteric filter is applied to a direct-view backlight is disclosed in Japanese Patent Application Laid-Open No. Hei 7-36032.

[0006]

The above prior art uses a cholesteric liquid crystal acting as a reflective polarizing plate,
It is intended to increase the luminance by reusing the reflected light. An optical path conversion element is applied to a liquid crystal display for a notebook personal computer in order to improve front luminance with low power consumption. The most commonly used optical path conversion element is BEF (trade name of 3M Company). This optical path changing element is applied to a lighting device having directivity and obtaining high-brightness display with low power consumption.

However, in the above prior art,
No mention is made of the polarization conversion efficiency when these light path conversion elements are applied to improve the front luminance. Furthermore, there is no mention of a configuration capable of improving the polarization conversion efficiency when an optical path conversion element is used.

As the optical path conversion element, a striped film (PET: polyethylene terephthalate) having a triangular cross section is generally used, and has biaxial birefringence. Therefore, it has been found that the polarization state changes when the incident linearly polarized light deviates from its optical axis, and as a result, the polarization conversion efficiency is reduced. It has been found that the polarization conversion efficiency is further reduced when two optical axes of this optical path conversion element are applied orthogonally.

SUMMARY OF THE INVENTION It is an object of the present invention to specify an optimal axial arrangement of an optical path conversion element and a polarizing plate when an optical path conversion element is applied in order to improve front luminance with low power consumption, and to achieve high polarization conversion efficiency. It is an object of the present invention to provide a bright liquid crystal display device.

Still another object of the present invention is to improve the light use efficiency and the front luminance by using a light guide having improved directivity while maintaining the polarization of the reflected light from the reflective polarizing plate. It is to provide a realized liquid crystal display device.

[0011]

The gist of the present invention to achieve the above object is as follows.

[1] In a liquid crystal display device having a liquid crystal display device for controlling polarization and a lighting device arranged on the back of the liquid crystal display device, a reflection means is provided in the lighting device, and an optical path is provided between the liquid crystal display device and the lighting device. And a reflection type polarization selecting unit, wherein the transmitted light of the reflection type polarization selecting unit and the reflected light from the reflection type polarization selecting unit are reflected by the reflection unit and again incident on the reflection type polarizing plate. A liquid crystal display in which the angle between the optical path conversion axis of the optical path conversion means and the polarization transmission axis of the reflection type polarization selection means is 0 to 30 degrees or 60 to 90 degrees so that the polarization states of the reflected lights are substantially the same. apparatus.

[2] The liquid crystal display device described above, wherein a birefringent medium is arranged on the back of the optical path changing means.

[3] A birefringent medium is arranged on the back of the optical path changing means, and the polarization axis of the birefringent medium is arranged at approximately 45 degrees with respect to the polarization direction of the reflected light from the reflection type polarization selecting means. Liquid crystal display.

[4] The liquid crystal display device as described above, wherein the birefringent medium disposed on the back side of the optical path changing means functions as a substantially 波長 wavelength plate.

[5] The liquid crystal display device as described above, wherein the angle formed by the optical path changing axis of the optical path changing means and the polarization transmission axis of the reflection type polarization selecting means is substantially parallel or orthogonal.

[6] The liquid crystal display element includes a liquid crystal layer sandwiched between a pair of transparent substrates, a liquid crystal alignment control electrode formed on the transparent substrate, and at least a transparent substrate on the display surface side of the transparent substrate. The liquid crystal display device having a polarizing plate, wherein the polarizing axis of the polarizing plate is disposed so as to be substantially parallel or substantially perpendicular to the polarization transmission axis of the reflection type polarization selecting means.

[7] The lighting device is a plate-shaped light guide,
A light source disposed around the light guide, wherein light emitted from the light source propagates through the light guide and is emitted from a light emission surface of the light guide; and a back surface of the light guide Is provided with an anti-slope surface composed of a number of concaves, convexities or steps having a fine inclined surface, the anti-slope surface has at least the inclined surface mirror-finished, and directly or an air layer on the back surface of the light guide. The liquid crystal display device described above, wherein a reflection plate is provided through the liquid crystal display.

[8] The liquid crystal display device as described above, wherein the optical path changing means is formed of an isotropic medium or a uniaxial birefringent medium.

[0020]

[9] The above-mentioned liquid crystal display in which the half-width, which is an angle range where the emission characteristic at the time of white display from the liquid crystal display device is の of the front luminance in the horizontal direction, is 60 degrees or less and 40 degrees or less in the vertical direction. apparatus.

That is, the polarization conversion efficiency is increased by preventing the reflected light from the reflection type polarizing plate from being distorted in the illumination device, and the directivity in at least one axial direction is increased. As a means for improving the omnidirectional directivity and improving the front luminance, the lighting device is a plate-shaped light guide,
Light emitted from a light source disposed around the light guide propagates through the light guide, and is emitted from the light emission surface of the light guide. An anti-slope surface having a large number of concaves, convexes or steps having a surface, wherein the anti-slope surface has a mirror surface at least at the inclined surface portion, and is reflected directly on the back surface of the light guide or through an air layer. A plate is provided.

The diffusion plate disposed on the light guide has a scattering characteristic while maintaining the polarization state of the transmitted light, for example, a hologram diffusion plate.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The above-mentioned lighting devices (backlights) are roughly classified into two types, a direct type and an edge light type. The direct type is a type in which the light source is inside the illuminating surface, and the edge light type is a type in which the light source is arranged outside the illuminating surface, and the light guide as the illuminating surface is made of a transparent acrylic resin.
A method in which a columnar luminous body such as a fluorescent lamp (cold-cathode discharge tube or hot-cathode discharge tube) is arranged on one or two sides, and a lamp cover made of a reflector is provided outside the luminous body to introduce light into the light guide. It is.

For a liquid crystal display that requires a low profile,
An edge-lit backlight is effective, and a direct-type backlight is effective for a liquid crystal display that is required to be lightweight and have a small frame.

In a conventional liquid crystal display device, an edge light type backlight is mainly used, and white ink is formed on the back surface of the light guide in order to obtain in-plane uniformity. In addition, a reflective polarizing plate is applied to improve light use efficiency.

As the reflective polarizing plate, USP 5,486,
No. 949 and SID92 Digest pp. 427.
No. 32 and Asia Display 95 Digest p
Cholesteric film disclosed in p.
There is a wave plate. Here, the former is a reflective polarizing plate type 1,
The latter will be referred to as a reflective polarizing plate type 2.

Here, the S-polarized light indicating the polarization state is a polarized light perpendicular to the incident surface (the incident surface is a plane formed by the incident light and the incident normal formed on the boundary surface), and the P-polarized light is incident on the incident surface. Parallel polarized light.

In general, a transparent medium having a refractive index N ₀ and a refractive index N ₁
When light is incident from the N ₀ medium to the N ₁ medium at the interface of the transparent medium, when the incident angle of the incident light is θ, the tangent of the incident angle θ is equal to N ₁ / N ₀ (tan θ = N _1). / N ₀ )
At this time, it is known that there is no reflection component of P-polarized light, all reflected light is S-polarized light, and transmitted light is the remaining S-polarized light and P-polarized light.

The incident angle θ at this time is called a Brewster angle. Utilizing this Brewster angle, media having different refractive indices are laminated, and the thickness of the laminated film is controlled on the wavelength order to control the phase of each polarized light, transmit only P-polarized light, and reflect S-polarized light. A reflective polarizing plate can be manufactured.

FIG. 5 shows an example of the reflection type polarizing plate type 1. This is a structure in which two kinds of transparent media having different refractive indexes are alternately laminated in a prismatic shape. The function of the reflective polarizing plate type 1 is to transmit only the P-polarized light 141 out of the non-polarized light 140 and reflect the S-polarized light 142 orthogonal thereto.

When a retardation plate is used as a depolarizer, the reflected S-polarized light 142 becomes elliptically polarized light (including linearly polarized light and circularly polarized light) by the retardation plate. And only the P-polarized component is transmitted,
The S-polarized component is reflected and returns to the light guide. By repeating this, almost all light is converted into P-polarized light 141 and emitted. Therefore, preferably, the reflected S-polarized light 142
Are converted to P-polarized light 141 so that 1
It is preferable to set a retardation plate acting as a quarter-wave plate so as to have a half wavelength.

On the other hand, an example of the reflective polarizing plate type 2 is shown in FIG.
Shown in The cholesteric liquid crystal polymer described in Asia Display 95 Digest p735 is laminated with a cholesteric liquid crystal polymer (22) having a different pitch so as to exhibit characteristic reflection in the visible wavelength region, and a certain rotation (clockwise or left) in the non-polarized light 143. (Circular), and reflects the counterclockwise (left-handed or right-handed) circularly-polarized light 145, and a quarter-wave plate 23 is laminated thereon.
4 is transmitted.

The function of the reflection type polarizing plate type 2 is to transmit only clockwise (or counterclockwise) circularly polarized light, reflect counterclockwise (or clockwise) circularly polarized light, and reduce the transmitted circularly polarized light by 1 /.
The light is converted into linearly polarized light in one direction by a four-wavelength plate.

On the other hand, the reflected counterclockwise (or clockwise)
The circularly polarized light is reflected by the specular reflector to become clockwise (or counterclockwise) circularly polarized light, passes through the reflective polarizing plate type 2, and becomes 1 /
The light is converted to linearly polarized light in one direction by the four-wavelength plate, and all light is converted to linearly polarized light. Even if the reflector is not a specular reflector, the reflected light becomes elliptically polarized light (including linearly polarized light and circularly polarized light), enters the reflective polarizing plate again, and transmits only clockwise (or counterclockwise) circularly polarized light. Circular (or clockwise) circularly polarized light is reflected back to the light guide.

By repeating this, almost all the light is converted into only clockwise (or counterclockwise) circularly polarized light, and thereafter, is output as linear polarized light in one direction by a quarter-wave plate.

Therefore, since there is considerable light absorption in the reflector, the reflected counterclockwise (or clockwise) circularly polarized light is converted into clockwise (or counterclockwise) circularly polarized light. In addition, it is preferable to use a perfect mirror reflection plate.

To clarify the difference between the structure and effects of the liquid crystal display device of the present invention and the conventional liquid crystal display device, first, a conventional liquid crystal display device will be described with reference to FIGS.
This will be described with reference to FIG.

As shown in FIG. 15, the conventional edge light type backlight is made of one transparent acrylic resin, and a light guide 65 having a white ink on the back is provided with a reflecting plate 64 on the back and a light guide. A light source 62 is provided on at least one of the 65 side surfaces.
And a diffusion plate 66 is arranged on the light exit surface of the light guide 65. Further, in order to increase the front luminance, the optical path conversion element 30 is arranged so as to be parallel or perpendicular to the long side of the light source 62.

The optical path conversion element 30 has a stripe shape having a triangular cross section as shown in FIG.

The liquid crystal display element 10 employs a TN mode having a 90-degree twist as the most general mode. At this time, the polarization transmission axis 11 of the lower polarizing plate and the polarization transmission axis 12 of the upper polarizing plate are: This is a so-called normally white mode arranged orthogonally. Therefore, the polarization transmission axis 21 of the reflective polarizer 20 is the same as the polarization transmission axis 11 of the lower polarizer.
And are arranged in parallel. That is, the stripe direction 31 of the optical path conversion element 30 (hereinafter, this stripe direction 31, that is, a fixed direction in which the optical path is not converted is referred to as the optical path conversion axis of the optical path conversion element) is the same as the polarization transmission axis 2 of the reflective polarizing plate 20.
Crosses 1 and 45 degrees.

When the reflective polarizing plate type 1 is applied as the reflective polarizing plate 20, in the above configuration, as shown in FIG. 13, the outgoing light 120 from the non-polarized light guide is transmitted through one linearly polarized light. The light 121 is transmitted, and the other orthogonal linearly polarized light is reflected by the reflective polarizing plate 20 to become reflected light 122.

In general, it has been found that the birefringent optical axis of the optical path conversion element 30 is in the direction of the optical path conversion axis 31. At this time, the reflected light 122, which is linearly polarized light, cannot maintain its polarization due to the birefringence of the optical path conversion element 30 because the polarization axis direction forms an angle of 45 degrees with the optical path conversion element 30, and the linearly polarized light becomes elliptically polarized light. become. Further, the light becomes non-polarized by the white ink on the back surface of the light guide 65 and the light diffusion of the diffusion plate 66, and is reflected by the reflection plate 64 to become the non-polarized light 123. Therefore, only the component parallel to the polarization transmission axis of the reflection type polarizing plate 20 is transmitted, and becomes the linearly polarized transmitted light 121A having the same polarization as the transmitted light 121. The reflected linearly-polarized reflected light 122A orthogonal to the linearly-polarized light of the transmitted light 121A becomes non-polarized light 123A through a process similar to that of the reflected non-polarized light 123. Then, it becomes the linearly polarized light transmission light 121B having the same polarization as the linearly polarized light transmission light 121, 121A.

Further, the reflected light 122B is
The non-polarized light 123B is obtained through the same process as that of 2,122A. By repeating the above process, in principle, all light is converted into the same linearly polarized light and emitted.

However, when the efficiency of the light emitted from the liquid crystal display device was actually measured, it was found that the amount of luminous flux only increased by about 30% with or without the reflective polarizing plate 20. Ideally, all unpolarized light is converted into linearly polarized light in one direction, and the light use efficiency becomes 100%.

The direct cause of the reduction in efficiency is that the reflection plate 64,
Absorption of the light guide 65, the white ink, the diffusion plate 66, and the like, and transmission of unnecessary polarized light due to imperfection of the reflective polarizing plate 20. In other words, the light absorption in each member is slight in one transmission reflection, but in the conventional configuration, polarization conversion cannot be performed efficiently by one reflection, so that many reflections and reflections are performed.
Polarization conversion takes place in transmission, resulting in increased absorption.

The above root cause is that the optical path conversion axis 31 of the optical path conversion element 30 and the polarization transmission axis 21 of the reflective polarizer 20 in FIG. Is converted into elliptically polarized light.
As a result, polarization is not efficiently converted by one reflection,
Since the polarization is converted by multiple reflections, the polarization conversion efficiency is reduced due to the large influence of the light absorption of each member.

When the reflective polarizing plate type 2 is applied as the reflective polarizing plate 22, in the above configuration, as shown in FIG. 11, the outgoing light 130 from the unpolarized light guide is converted into one circularly polarized light. Is transmitted and becomes linearly polarized light 131 by the phase difference plate 23 and is transmitted. The reflected light 132 that is the other circularly polarized light is reflected by the reflective polarizing plate 22.

At this time, due to the birefringence of the optical path conversion element 30, the reflected light 132 that is circularly polarized light becomes elliptically polarized light without maintaining its polarization. Further, the white ink on the back surface of the light guide 65 and the light diffusion of the diffusion plate 66 make the light non-polarized,
And becomes unpolarized light 133. Accordingly, one circularly polarized light is transmitted by the reflective polarizing plate 22, becomes the same linearly polarized light 131A as the linearly polarized light 131 by the phase difference plate 23, and the other reflected circularly polarized light 132A is reflected by the retardation plate 23, and the reflected light 1 is reflected.
Through a process similar to that of No. 32, a non-polarized light 133A is obtained. Similarly, linearly polarized light 131B, reflected light 132B, and unpolarized light 13B
3B.

Therefore, in principle, all light is converted into the same linearly polarized light. However, as in the case of using the reflective polarizing plate type 1, the light use efficiency can be improved by at most 30%.
Only an improvement. The cause is light absorption loss due to multiple reflections as in the case of the reflective polarizing plate type 1. In the case of the reflective polarizing plate type 2, since the circularly polarized light is reflected, the optical path conversion element 30 is not used. It is preferable to use an isotropic medium having no birefringence, or to arrange a retardation plate so as to be orthogonal or parallel to the optical path conversion axis 31 before the reflected light enters the optical path conversion element 30.

Conventionally, in order to further improve the front luminance, a configuration is adopted in which the respective optical path conversion axes 31, 33 of the optical path conversion elements 30, 32 are orthogonally arranged as shown in FIG. As a result, the front luminance can be increased by giving directivity in substantially all directions, instead of having directivity in one axis direction (horizontal or orthogonal direction) in one sheet.

The conventional edge light type backlight has the following features.
A reflector 64 is arranged on the back surface of a light guide 65 made of a sheet of transparent acrylic resin and having white ink on the back surface, and a light source 62 is arranged on at least one of the side surfaces of the light guide 65. A diffusion plate 66 is disposed on the surface.

Further, the light source 62 is arranged parallel or perpendicular to the long side. The liquid crystal display element 10 employs a TN mode having a 90-degree twist as the most general mode. At this time, it is a so-called normally white mode in which the polarization transmission axis 11 of the lower polarizer and the polarization transmission axis 12 of the upper polarizer are arranged to be orthogonal to each other. Therefore, the polarization transmission axis 21 of the reflection type polarizing plate 20 is arranged parallel to the polarization transmission axis 11 of the lower polarizing plate. That is, the stripe direction of the optical path conversion elements 30 and 32, that is, the optical path conversion axis 3
Reference numerals 1 and 33 are parallel or orthogonal to the polarization transmission axis 21 of the reflective polarizing plate 20.

Also in this configuration, similar to FIG. 15, the light use efficiency was only improved by about 30% by applying the reflective polarizing plate. In this configuration, when the reflective polarizing plate type 2 is used as the reflective polarizing plate 20, it is necessary to dispose a retardation plate immediately before the optical path conversion element 30 to make it linearly polarized. Even when using No. 1, the light use efficiency was improved by about 30% at most.

The reason is that the optical path conversion elements 30 and 32
Is a biaxial anisotropic medium, which causes a change in the polarization state even if the projected component of the optical axis is parallel or orthogonal to the incident linearly polarized light. This is because the effect of the change in the polarization state is small when one sheet is used, but is enhanced by using two sheets. The reason for this increase is that when the apex angle of the optical path conversion element 30 is 90 degrees, the vertically incident light is totally reflected and not emitted, so that two sheets are used to repeatedly perform multiple reflections and change the polarization state. Greatly reduces the efficiency.

As described above, it was found that when a reflective polarizing plate and an optical path changing element were used to improve the light use efficiency and the front luminance, the light use efficiency could not be improved due to multiple reflections. Therefore, the principle of efficiently reusing reflected light by one reflection according to the present invention will be described with reference to FIGS.

First, the operation when the reflective polarizing plate type 1 is applied as the reflective polarizing plate 20 is shown in FIG. One linearly polarized light 1 of the unpolarized light 120 which is the light emitted from the light guide.
The other linearly polarized light 122 that passes through and is orthogonal to the transmitted light 121 is reflected by the reflective polarizing plate 20 and is converted into circularly polarized light 124 by the birefringent medium 40 acting as a quarter-wave plate. Further, the circularly polarized light 124 is specularly reflected by the reflection plate 61 and becomes a circularly polarized light 125 which is opposite to the circularly polarized light 124. The circularly polarized light 125 becomes the same linearly polarized light 126 as the transmitted light 121 in the birefringent medium 40, passes through the reflective polarizing plate 20, and transmits the transmitted light 1
21C. By this process, all the light is converted into the same linearly polarized light by one reflection, and the polarization conversion with higher efficiency can be achieved.

FIG. 12 shows the operation when the reflective polarizing plate type 2 is applied as the reflective polarizing plate 22. Out of the unpolarized light 130 emitted from the light guide, one circularly polarized light is transmitted, and becomes a linearly polarized light 131 by the birefringent medium 40 acting as a quarter-wave plate. On the other hand, the other reflected circularly polarized light 132 is specularly reflected by the reflecting plate 61 and is reflected by the circularly polarized light 1.
It becomes circularly polarized light 136 that is opposite to 32. Circularly polarized light 132
The light passes through the reflective polarizing plate 22 and is reflected by the birefringent medium
1 and becomes the same linearly polarized light 131C and is emitted.

By this process, all the light is converted into the same linearly polarized light by one reflection, and the polarization conversion can be achieved with higher efficiency. When using this type 2 reflective polarizer, it is necessary to convert linearly polarized light before entering the optical path conversion element, or to apply a uniaxial anisotropic or even isotropic medium at least for the optical path conversion element. preferable. When a uniaxial anisotropic medium is used as the optical path conversion element, it is preferable that the optical path conversion element functions as a quarter-wave plate so that linearly polarized light becomes circularly polarized light after transmission.

As described above, in order to efficiently perform polarization conversion by one reflection, it is necessary to arrange the optical path conversion elements so as not to be affected by birefringence. In addition, it is optimal for the efficiency improvement to maintain the polarization of the light guide, the diffusion plate, and the like. That is, when a reflection type polarizing plate is used, the polarization maintaining performance of the diffusion plate becomes important. In particular, it is preferable to use a hologram diffusion plate as a diffusion plate which does not lose polarization. For example, a hologram diffusion plate of SID93 digest pp29 or SPIEvol1536 (199
1) A light control film of pp138 can be used. In addition, in the case where the directivity is enhanced in all directions and the front luminance is improved, conventionally, two optical path conversion elements 30 are used.
It was found that when two sheets were used, the efficiency decreased due to multiple reflection. Therefore, it is effective to increase the directivity of the light path conversion axis direction with a light guide.

An example of the light guide is shown in FIGS. In order to reflect the reflected light from the reflective polarizing plate back to the liquid crystal display element while maintaining the polarization of the reflected light, the rear surface of the light guide 60 in FIG.
0B, and a specular reflection plate 61 is provided on the back surface of the light guide. At this time, the area ratio of the inclined surface is made smaller than that of the flat portion. The inclined surface is a surface for emitting light from the light guide, the mirror surface is a reflection surface, and the flat portion is for totally reflecting and propagating the light in the light guide. Although the inclined surface and the flat surface may be metal reflecting surfaces, it is preferable to use total reflection having the highest reflectance since the number of reflections when propagating in the light guide increases. Further, as shown in FIG. 8, a flat portion 60D which is slightly inclined and an inclined portion 60C can be provided. With this configuration, most of the light reflected from the reflective polarizer passes through the flat portion on the back surface of the light guide, and is emitted again from the light guide while maintaining the polarization state substantially by the reflector disposed on the back surface. This makes it possible to use light efficiently with little absorption by the incident-side polarizing plate of the liquid crystal display element, and to improve brightness.

When the incident light 153 (163) from the light source enters the flat portion 60B (60D) on the back surface of the light guide 60, the totally reflected light 154 (164) propagates through the light guide 60 and becomes fine. Only when the light enters the mirror surface portion 60A (60C), the light is emitted from the light guide as the output light 156 (166), or propagates through the light guide and propagates the light 151 (161, 161).
162). In addition, the total reflection light 15
2,155 (164).

At the surface of the light guide 60, light having an incident angle equal to or greater than the total reflection angle θc determined by the refractive index of the light guide is totally reflected and propagates through the light guide. Light having an incident angle smaller than the total reflection angle θc is refracted by the upper surface of the light guide and emitted. For example, air (refractive index n = 1) and a transparent resin such as acrylic, polycarbonate, polyurethane, and polystyrene (refractive index n = 1.
The total reflection angle θc at the interface of about 5) is θc = sin
^{1 1} (1 / n) = 42 °. The light θ incident on the light guide becomes light within − (90 ° −θc) ≦ θ ≦ + (90 ° −θc), and is totally reflected on the flat portion on the upper surface or the lower surface of the light guide.

Further, an important configuration of the present invention is that one axis direction is a light guide, and a direction orthogonal to the light guide is an optical path changing element to improve the front luminance. FIG. 9 shows an optical path conversion axis 31 of the optical path conversion element, a polarization transmission axis 31A of the reflective polarizing plate, and an angle 31B formed by the axis.

The optical path changing element 30 is a uniaxial birefringent medium,
FIG. 10 shows an example of the change in the polarization conversion efficiency 200 and the rate of change 201 of the polarization conversion efficiency 200 due to the angle formed when the light acts as a substantially quarter-wave plate at normal incidence.

FIG. 10 shows that the polarization conversion efficiency is lowest at an angle of 45 degrees and the change is steep. Therefore, when the angle formed is 45 degrees, the efficiency changes greatly due to a shift in the arrangement angle of the polarizing plate or a shift in the liquid crystal rubbing angle. Taking the manufacturing margin into consideration, the above-mentioned angle of 45 degrees is the worst condition. The optical path conversion element has birefringence, and its optical axis exists in the optical path conversion axis direction. Therefore, in order to reduce the change in the polarization state due to the birefringence of the optical path conversion element, the angle between the direction of the optical path conversion axis and the direction of polarization of the reflected light from the reflective polarizer is set so as not to be 45 degrees.

It is desirable that the angle between the direction of the optical path conversion axis and the direction of polarization of the reflected light from the reflective polarizer is 0 to 30 degrees or 60 to 90 degrees. That is, the direction of polarization from the mold-type polarizing plate is orthogonal or parallel.

As described with reference to FIGS. 12 and 14, when the optical path conversion element 30 has no birefringence, conversion can be efficiently realized. However, the optical path conversion element 30 is generally a birefringent medium.

Therefore, the optical path conversion element 30 is made of a uniaxial birefringent medium and functions as a substantially 波長 wavelength plate.
0 optical path conversion axis 31 and the polarization transmission axis 31A of the reflective polarizing plate
When the angle 31B is 45 degrees, the polarized light reflected by the reflective polarizing plate is reflected by the reflecting plate on the back surface and passes through the optical path conversion element twice. As a result, it is shifted by 90 degrees from the desired linearly polarized light due to the phase change, and cannot be reused at all by one reflection. Accordingly, the light use efficiency is reduced as shown in FIG.

However, the angle 31B between the optical path conversion axis 31 and the polarization transmission axis 31A of the reflective polarizer is defined as
When the angle is set to 0 degree or 90 degrees, the reflected light from the reflective polarizing plate can be efficiently reused by one reflection.

Further, it was found that when the rate of change of the efficiency was set to 0.5 or less, the in-plane variation of the birefringence of the optical path conversion element 30 could not be visually recognized. In order to reduce the rate of change in efficiency to 0.5 or less, the angle 3
1B needs to be 0 to 30 degrees or 60 to 90 degrees.

As described above, it is possible to obtain a liquid crystal display device having a high polarization conversion efficiency and a high light use efficiency realizing an improvement in frontal luminance.

[0072]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a liquid crystal display device of the present invention will be described in detail.

FIG. 1 is a schematic perspective view showing one embodiment of the liquid crystal display device of the present invention. In this embodiment, a light guide 6 made of a transparent acrylic resin is used by using an edge light type illumination device.
Using 0, a cold cathode fluorescent lamp 62 having an emission length corresponding to the length on one side surface and a lamp cover (not shown) for covering the cold cathode fluorescent lamp 62 and reflecting light toward the light guide are provided on the end surface. On the back surface of the light guide 60, a reflection plate 61 was arranged.

The light source 62 has a tube diameter of 2.6 mm.
A length of about 290 mm (in the depth direction of the paper) was used.
As the lamp cover, a cylindrical or elliptical reflecting plate surrounding the cold cathode fluorescent lamp is used. As the light guide 60, the refractive index is 1.49 and the size is 290 mm × 2.
The one having a size of 25 mm × 3.5 mm was used.

Fine grooves were formed on the back surface of the light guide 60, and uniformity was maintained by changing the pitch of the grooves. Also,
A diffusing plate 63 made of a hologram that does not break the polarization was disposed on the emission side of the light guide 60. Further, a retardation plate 41 made of polycarbonate and having a retardation of 130 nm is used.
The extension axis 41A is the optical path conversion axis 3 of the optical path conversion element 30.
1 and 45 degrees.

As the reflection type polarizing plate 20, a reflection type polarizing plate type 2 was used, and its polarization transmission axis 21 was arranged to be orthogonal to the optical path conversion axis 31.

The liquid crystal display element 10 uses a 90 ° twisted TN liquid crystal, and is arranged such that the transmission axes of the polarizing plates are perpendicular to the back surface 11 and the front surface 12, respectively. The optical path conversion element 30 had biaxial birefringence, and the projected component of the optical axis was parallel to the optical path conversion axis 31.

As the liquid crystal display element 10, the one shown in the schematic sectional view of FIG. 2 was used. Transparent electrodes 13A, 13 inside
B, an alignment film 14A, 1 having a rubbed thereon.
4B and the liquid crystal layer 1 between the upper and lower substrates 12A and 12B.
5 is a normally white mode liquid crystal display element in which the polarization transmission axis is orthogonal to both sides thereof. Here, the liquid crystal display mode is not limited as long as the display mode modulates polarization.

With the above configuration, when the incident light 100 to the light guide 60 is incident on the fine structure on the back surface of the light guide as shown in FIG. It is emitted from the line direction. Unpolarized outgoing light 10
Reference numeral 1 denotes one linearly polarized transmitted light 1 by the reflection type polarizing plate 20.
02 is transmitted, and the linearly polarized light orthogonal to the linearly polarized transmitted light 102 is reflected and becomes reflected light 104.

The reflected light 104 is converted into circularly polarized light 103 by the birefringent medium 40 without being changed in the polarization state by the optical path conversion element 30, and reflected by the reflecting plate 61 disposed on the back surface of the light guide 60. Is done. The reflected light 105 reflected is
The light becomes circularly polarized light in the opposite direction to the circularly polarized light 103 and enters the birefringent medium 40 again. The light 106 transmitted through the birefringent medium 40 acting as a 波長 wavelength plate becomes the same linearly polarized light as the transmitted light 102, transmits through the reflective polarizing plate 20, and becomes the output light 107. As described above, since the polarization conversion is efficiently performed by one reflection, the light use efficiency can be greatly improved.

Next, FIG. 4 is a schematic cross-sectional view when viewed from a direction orthogonal to the optical path conversion axis 31 of the optical path conversion element 30 from a different viewpoint.

The incident light 111 from the light guide 60 is directed in the vertical direction by the optical path conversion element 30 to become transmitted light 112. The unpolarized transmitted light 112 is reflected by the reflective polarizing plate 20.
Then, only one of the linearly polarized light is transmitted, and the linearly polarized light is transmitted.

The reflected light 114 becomes linearly polarized light orthogonal to the transmitted light 113 and enters the optical path conversion element 30 again. As a result, the transmitted light 115 is transmitted obliquely through the birefringent medium 40, and the birefringent medium 40 acts as a quarter-wave plate in this optical path.

The transmitted light 115 becomes circularly polarized light,
The reflected light is converted into circularly polarized light 116 in the opposite direction to the transmitted light 115, passes through the birefringent medium 40 again, and is transmitted through the transmitted light 11.
3 becomes the same linearly polarized light 117. As a result, the light passes through the reflective polarizing plate 20 and becomes the same linearly polarized light transmission light 118 as the linearly polarized light transmission light 113.

As described above, the polarization conversion is efficiently performed by one reflection, and the front luminance and the light use efficiency can be improved by the directivity. As a result, the conventional light use efficiency was improved by about 30% by using the reflective polarizing plate, whereas the present embodiment was improved by about 60%. Furthermore, when a polycarbonate which is a uniaxial anisotropic medium was applied to the optical path conversion element, the light use efficiency was further improved.

Next, when a reflection type polarizing plate type 2 comprising a cholesteric film and a quarter-wave plate is applied as the reflection type polarizing plate 20, in order to obtain the same effect as described above, the reflection type polarizing plate 20 is provided below the cholesteric film. A quarter-wave plate was also arranged. Otherwise, the configuration is the same as above. As a result, the conventional light utilization efficiency is improved by about 30% by using the reflective polarizing plate, whereas the present embodiment is improved by about 5%.
0% improvement.

Further, with the above structure, in-plane uniformity of 90% (minimum luminance ratio to maximum luminance) was achieved. Furthermore, as for the spread of the emitted light, as a result of measuring the angle range of the brightness in which the brightness is reduced to に 対 し て with respect to the front brightness, the spread of the emitted light is It was about ± 20 degrees in the orthogonal direction. Therefore, the directivity can be enhanced in all directions with one optical path conversion element, and the front luminance is dramatically improved.

The cholesteric film used in the present invention utilizes a characteristic reflection using a cholesteric liquid crystal polymer. The cholesteric liquid crystal polymer includes layers having different pitches so that the characteristic reflection is exhibited in a visible region. Laminated ones are preferred.

Experimentally, it was confirmed that at least two layers were required. There is no problem if the pitch of the cholesteric liquid crystal polymer changes in the single layer or if Δn (refractive index anisotropy) is sufficiently large.

Further, 1 /
A retardation plate acting as a four-wavelength plate was arranged. As a result, the unpolarized incident light is reflected by the circularly polarized light component in the same direction as the cholesteric liquid crystal polymer, transmitted through the counter-circularly polarized light component, and is output as linearly polarized light by the phase difference plate. Therefore, a polarizing plate having no light absorption loss can be manufactured, and the reflected circularly polarized light can be reused as, for example, counter-circularly polarized light by metal reflection, thereby improving the light use efficiency.

In the above embodiment, the light source lamp is arranged on the side of the light guide. Further, the pitch, angle, and height of the microstructure on the back surface of the light guide are fixed, but can be changed according to the distance from the light source lamp.

As a result of studying the above, the pitch is 50
0 μm or less, the angle is preferably 30 to 50 degrees, and the height is preferably 50 μm or less. In addition, the microstructure can be any irregularities,
Further, the fine structure may be a curved surface.

In the above embodiment, a display mode in which a vertical electric field of the TN liquid crystal layer is applied has been described as an example. However, in a display mode in which polarization is controlled, a display such as a TN liquid crystal, a STN liquid crystal, an MVA method, or the like, to which a horizontal electric field is applied. The mode is not specified.

[0094]

According to the present invention, the liquid crystal display device for controlling the polarization and the lighting device of the liquid crystal display device provided with the lighting device on the back of the device are provided with a reflecting means, and an optical path changing means between the liquid crystal display element and the lighting device. A reflection-type polarization selector, and the optical path conversion is performed so that when the transmitted light of the reflection-type polarization selector and the reflected light from the reflection-type polarization selector again enter the reflection-type polarizing plate, they have substantially the same polarization state. By defining the direction of the optical path conversion axis of the means and the polarization transmission axis of the reflective polarization selecting means, a liquid crystal display device with low power consumption and high front luminance can be realized.

In particular, the angle between the direction of the optical path changing axis of the optical path changing means and the polarization direction of the reflected light from the reflection type polarization selecting means is desirably 0 to 30 degrees or 60 to 90 degrees. The liquid crystal display device having low power consumption and high front luminance can be realized by arranging the conversion axis direction and the reflected light polarization direction to be orthogonal or parallel.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing one embodiment of a liquid crystal display device of the present invention.

FIG. 2 is a schematic sectional view showing one embodiment of a liquid crystal display element applied to the present invention.

FIG. 3 is a schematic sectional view showing one embodiment of the liquid crystal display device of the present invention.

FIG. 4 is a schematic sectional view showing one embodiment of the liquid crystal display device of the present invention.

FIG. 5 is a schematic cross-sectional view showing one embodiment of a reflective polarizing plate applied to the present invention.

FIG. 6 is a schematic cross-sectional view showing one embodiment of a reflective polarizing plate applied to the present invention.

FIG. 7 is a schematic sectional view showing one embodiment of a lighting device applied to the present invention.

FIG. 8 is a schematic sectional view showing one embodiment of a lighting device applied to the present invention.

FIG. 9 is a diagram illustrating a part of the configuration of the present invention.

FIG. 10 is a graph showing a relationship between the polarization direction of the present invention and the light efficiency with respect to the angle formed by the route change element.

FIG. 11 is a schematic diagram illustrating a polarization state of light of a conventional liquid crystal display device.

FIG. 12 is a schematic diagram illustrating a polarization state of light of the liquid crystal display device of the present invention.

FIG. 13 is a schematic diagram illustrating a polarization state of light of a conventional liquid crystal display device.

FIG. 14 is a schematic diagram illustrating a polarization state of light of the liquid crystal display device of the present invention.

FIG. 15 is a schematic sectional view showing an example of a conventional liquid crystal display device.

FIG. 16 is a schematic sectional view showing an example of a conventional liquid crystal display device.

[Explanation of symbols]

10: liquid crystal display element, 11: polarization transmission axis of lower polarizing plate,
Reference numeral 12 denotes a polarization transmission axis of an upper polarizing plate, 11A and 11B denotes a polarizing plate, 12A and 12B denotes a substrate, 13A and 13B denotes a transparent electrode, 14A and 14B denotes an alignment film, and 15 denotes a liquid crystal layer.
2 ... Reflection type polarizing plate, 21 ... Polarization transmission axis, 23 ... Phase plate, 24 ... Dielectric multilayer film, 25 ... Reflection type polarization plate (Type 1), 26 ... Reflection type polarization plate (Type 2), 30, 32 ...
Optical path conversion elements, 31, 33: Optical path conversion axis of optical path conversion element, 31A: Polarization transmission axis of reflective polarizing plate, 40: Birefringent medium, 41: Retardation plate, 41A: Stretching axis, 60, 65 ...
Light guide, 60A, 60C: mirror part, 60B, 60D: flat part, 61, 64: reflector, 62: light source, 63, 6
6 ... Diffusion plate, 100, 111, 120, 130, 15
0, 153, 160, 163 ... incident light, 103, 11
6,124,125,136,145 ... circularly polarized light, 10
1, 107, 156, 166 ... outgoing light, 102, 11
3, 118, 121, 121A, 121B, 144 ... linearly polarized transmitted light, 104, 105, 114, 122, 12
2A, 122B: reflected light, 112, 115: transmitted light, 1
23, 123A, 123B140, 143 ... non-polarized light, 1
17, 126 ... linearly polarized light, 131, 131A, 131
B, 131C, 106 ... linearly polarized light, 132, 132A,
132B: reflected light, 133, 133A, 133B: unpolarized light, 151, 161, 162: propagated light, 152, 15
4,155,164,165 ... Totally reflected light.

Continued on the front page (72) Inventor Makoto Tsumura 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi Ltd. (Reference) 2H091 FA08Z FA11X FA11Z FA14Z FA19Z FA23Z FA31Z FA41Z FD01 FD07 LA16 5C094 AA10 AA22 BA44 DA12 ED14 ED20 JA09

Claims (9)

[Claims] 2. A liquid crystal display device comprising: a liquid crystal display device for controlling polarization; and a liquid crystal display device having an illumination device disposed on the back of the liquid crystal display device. And a reflection type polarization selection unit, and the transmitted light of the reflection type polarization selection unit and the reflection light from the reflection type polarization selection unit are reflected by the reflection unit and again enter the reflection type polarization plate. The angle formed between the optical path conversion axis of the optical path conversion means and the polarization transmission axis of the reflection type polarization selection means is zero so that the polarization state of the reflected light is substantially the same.
A liquid crystal display device having an angle of 30 to 30 degrees or 60 to 90 degrees. 2. The liquid crystal display device according to claim 1, wherein a birefringent medium is arranged on a back surface of said optical path changing means. 3. A birefringent medium is arranged on the back of said optical path changing means, and the polarization axis of said birefringent medium is arranged at substantially 45 degrees with respect to the polarization direction of the reflected light from said reflection type polarization selecting means. 3. The liquid crystal display device according to 1. 4. The liquid crystal display device according to claim 2, wherein the birefringent medium disposed on the back surface of the optical path changing means functions as a substantially quarter-wave plate. 5. The liquid crystal display device according to claim 1, wherein an angle between an optical path conversion axis of the optical path conversion unit and a polarization transmission axis of the reflection type polarization selection unit is substantially parallel or orthogonal. 6. A liquid crystal display device comprising: a liquid crystal layer sandwiched between a pair of transparent substrates; a liquid crystal alignment control electrode formed on the transparent substrate; and a polarized light on at least a display surface side transparent substrate of the transparent substrate. The liquid crystal display device according to claim 1, further comprising a plate, wherein a polarizing axis of the polarizing plate is disposed so as to be substantially parallel or substantially perpendicular to a polarization transmission axis of the reflection type polarization selecting unit. 7. The lighting device includes a plate-shaped light guide and a light source disposed around the plate-shaped light guide, and light emitted from the light source propagates through the light guide and emits light from the light guide. The light guide is configured to be emitted from a surface, and the back surface of the light guide includes an anti-slope surface formed of a number of concaves, protrusions, or steps having a fine inclined surface, and the anti-slope surface is at least a mirror surface. The liquid crystal display device according to claim 1, wherein a reflection plate is provided on the back surface of the light guide directly or via an air layer. 8. The liquid crystal display device according to claim 1, wherein said optical path changing means is made of an isotropic medium or a uniaxial birefringent medium. 9. An emission characteristic of the liquid crystal display device during white display has a half value width of 60 degrees or less in an angle range where the front luminance is １ ／ in the horizontal direction and 40 degrees or less in a vertical direction. 6. The liquid crystal display device according to any one of 1 to 5.